In the oil and gas industry many downhole operations rely on objects dropped from surface to be caught at some point along a downhole system to perform a desired function, such as mechanical actuation, flow diversion or the like. For example, International Applications Publication Nos. WO 2011/117601 and WO 2011/117602 each disclose downhole apparatus which catch objects for use in actuation of a downhole tool and flow diversion within the tool, for example for use in fracturing operations.
To catch an object, a catching apparatus typically defines a restriction along the associated flow path. However, it is undesirable to permanently create a restriction, as this restriction in the flowpath may cause detrimental effects to the operation of the wellbore. As such, a significant challenge in the industry has been to develop a catching apparatus which can establish the necessary restriction on a temporary basis.
A collet-type catching apparatus is known which includes a number of collet fingers each carrying a collet or seat member. The collet fingers are capable of being deflected radially to move the collet members between radially extended (outwards) and retracted (inwards) configurations. When radially retracted the collet members create a restriction and permit an object to be caught.
The catching apparatus may be installed in the downhole system with its collet members in a radially extended (outwards) configuration. In use, when actuation of a tool is required, the catching apparatus is actuated, e.g. by longitudinal movement relative to a sleeve, such that the collet members move into a retracted (inwards) configuration.
To provide the catching apparatus having collet members in a radially extended (outwards) configuration, the collet members are typically provided in a plastically deformed state. That is, during manufacture of the catching apparatus, the collets are plastically deformed outwardly to force them in the extended configuration, ready to be deployed in a downhole system.
In use, depending on the position of each catching device in a series of catching devices along the wellbore, many of the catching devices may be subjected to the passage of a large number of actuating objects, e.g. balls, before being deployed into their retracted (catching) configuration. The actuating objects typically travel at high velocity, carrying high momentum and energy. As they travel down the wellbore, the actuating objects will cause high impact collisions with the various parts of the downhole system. Therefore, the passage of each actuating object may cause high energy impacts or shocks on the extended collet members of the catching apparatus.
It has been observed that repeated shocks caused by the actuating objects on the collet members tend to cause the collet members to progressively move inwards, e.g. towards their retracted pre-deformed configuration. This is undesirable for several reasons. Firstly, if the collet members move inwards into the flow path of a fluid pumped into the wellbore, the collet members are directly exposed to the fluid flow path, and are therefore more likely to suffer from premature erosion. This is particularly true when the injected fluid is an abrasive fluid, for example a fracturing fluid comprising solid proppant particles. This may cause the collet members to lose their required sealing ability when actuated into their retracted configuration. Secondly, as the collet members move inwards into the flow path, the passage of each actuating object causes an increasingly higher impact on the collet fingers, thus exacerbating this adverse effect. Frictional collision of the collet members with the actuating objects may also cause further undesirable erosion, both of the collet members and of the actuating object.